Gas-liquid mixing systems are conventionally used for various purposes. For example, oxygen is often used to treat liquids such as sewage and hydrometallurgical process liquids, while hydrogen is used to treat various types of unsaturated organic liquids.
Waste water treatment is one particular application where gas-liquid mixing systems are used. Industrial and municipal waste water is often treated by biotreatment techniques in which aerobic organisms convert contaminants into environmentally safe substances. Sufficient oxygen must be provided to the aerobic organisms in order to carry out the biotreatment process. Accordingly, oxygen delivery systems are conventionally used to dissolve either pure oxygen or oxygen from air in the water being treated. In order to be economically feasible, most of the injected pure oxygen must dissolve in the waste water because either the cost of oxygen or the cost of power to aerate with air typically represents the highest operating cost of the system.
One type of conventional gas-liquid mixing system is typically referred to as the Advanced Gas Reactor (AGR) system. Such AGR systems incorporate an open ended hollow draft tube in a mixing vessel. An impeller is positioned within the hollow draft tube in order to draw liquid into the top of the hollow draft tube for discharge at the bottom thereof to produce a recirculating flow path in the mixing vessel. Vortices are formed in the inlet area of the draft tube in order to draw feed gas from the overhead space above the liquid into the recirculating liquid passing downward through the draft tube.
U.S. Pat. No. 4,328,175 to Roeckel et al. discloses a vortex mixer including a draft tube with a conical inlet section. The inner surface of the inlet cone includes two to four small baffles which are said to prevent bulk rotation of liquid in the vessel during rotation of the mixer impeller.
U.S. Pat. Nos. 4,454,077 and Re. 32,562 to Litz disclose a vortex mixer including an axial impeller and several additional structural features which are said to improve gas utilization efficiency. In particular, efficiency is said to be increased by providing protuberances or indentations on the axial impeller shaft, providing vertical baffles in the draft tube below the axial impeller, and providing a radial flow impeller on the impeller shaft between the blades of the axial impeller and the vertical baffles. The draft tube includes a conical inlet section which forms an angle of from about 145 to about 175.degree. with the outside vertical wall of the draft tube. Vertical inlet guide baffles are mounted in the conical inlet.
U.S. Pat. No. 5,009,816 to Weise et al. discloses a vortex mixer comprising multiple stacked mixers similar to those disclosed in U.S. Pat. Nos. 4,454,077 and Re. 32,562. The mixer likewise includes turbulence-promoting protuberances on the axial impeller shaft and a radial flow impeller mounted below the blades of the axial impeller.
The disclosure of each patent cited above is incorporated herein by reference.
Although attempts have been made to increase the efficiency of vortex mixers, the above-noted mixers require large amounts of pumped liquid and multiple recirculation passes of liquid and gas through the mixer in order to achieve sufficient gas dissolution. This leads to a large amount of energy input to achieve the desired amount of gas dissolution. The present invention has been developed in view of the foregoing, and to remedy other deficiencies of the prior art.